It is already known to use the technical possibilities of the secondary control in the dynamic range in test equipment for engines. Please see in this context the case of a pure speed control as it is disclosed in "The Hydraulic Trainer", volume 6, pages 59 through 61, published by Mannesmann Rexroth GmbH, Lohr am Main, Germany.
As far as the problems are concerned with respect to said known test equipment it was not only considered to be necessary to measure and evaluate technical data, like the pressure, the volume flow, the temperature, the force, the speed, the acceleration and the gas composition, but it was further considered to be necessary to carry out dynamic tests for a long period of time, with the load being automatically changed and conditions being provided as they occur during road tests. For this kind of test operation the dynamic control of speed and torque and the measurement of said values is characteristic. The above-mentioned document discloses in detail a test cell together with a load machine, comprising a distribution gear means and two speed-controlled axial piston machines which are arranged in parallel. The test object is coupled thereto via a torque measuring hub. The use of a torque measuring hub, however, has the distadvantage, that it is relatively expensive. Further, the torque measurement hub cannot be subjected to dynamic loads and the hub needs to be replaced at an early stage, for instance, after about 500 hours of operation. In case a torque measurement hub brakes down during a test, the entire test has to be started again. Such a situation is clearly undesirable. Attention is drawn to the fact that the above-mentioned reference refers to the term "impressed operating pressure" at page 15 in the left column, a term which will be used below.
It is also considered to be prior art in the field of automobiles to use in test equipment for combustion engines direct current engines which are supplied via current controllers. In the course of such tests, the energy generated by the internal combustion engines is fed into the electric motor and is fed from there back into the mains. This requires a significant amount of control equipment. Also, because of the high mass inertia the dynamics of such test equipment does not fulfil many requirements. The suggestion to use converter-fed current machines having a speed-torque-characteristic similar to those of direct current machines, does not help even if one considers that the mass inertia moments can be reduced. Thus, even though the purely electric solution does not require a torque measurement hub, because the torque can be calculated from the current which is received or supplied, the above-mentioned distadvantages result.
DE-28 48 595 A1 discloses a method and apparatus for controlling the output torque of a gear means together with two hydrostats, a primary hydrostat and a second hydrostat, which are flow coupled by volume. The use of said apparatus requires an intrusion at the primary hydrostat for the purpose of changing the .DELTA.p for the control of the torque. This means, that the hydraulic spring formed by said apparatus has a disadvantageous result with respect to the dynamic of said apparatus.